Over-expression of receptor tyrosine kinases (RTK) by tumor cells is associated with resistance to ionizing radiation and other cytotoxic agents. The most studied RTK with respect to radiation resistance are members of the ERBB family, especially epidermal growth factor receptor (EGFR) and the insulin-like growth factor-I receptor (IGF-1R). EGFR activation by radiation initiates a major cytoprotective response mediated, in part, by the MAPK signal transduction pathway. In addition to cytoprotective pro-proliferative radiation response of carcinoma cells, an anti-apoptotic response through STAT3/BCL-XL and AKT is regulated, at least in part, through RTK and non-RTKs involving EGFR, IGF-1R and SRC. The regulatory aspects of these interactions, including potential transactivation and compensatory mechanisms, are the primary focus of studies proposed in Project 1. The impact of these studies will be enhanced because the receptors signal through RAS, a major focus for Project 2. Recent studies have also established links between RTKs and proteins (ATM and DNA-PK) involved in DNA damage repair. In collaboration with Project 3, these links will be investigated. The Specific Aims of Project 1 are: (1) Interactions between EGFR and IGF-1R will be quantified in terms of expression levels, downstream kinase activity profiles, compensatory expression and functional changes in one receptor upon inhibiting the other, transactivation mechanisms operating between ERBB molecules and IGF-IR, and in vitro and in vivo radiosensitization studies with dominant negative mutants of EGFR and IGF-IR singly and combined; (2) Receptor modulation of anti-apoptotic pathways are investigated in terms of anti-and pro-apoptotic protein expression, coordinated signaling along the EGFR (or IGF-IR) to BCL-XL pathways resulting in anti-apoptotic responses, RTK-->PI3K-->AKT anti-apoptotic signal transduction, and receptor modulation of anti-apoptotic mechanisms and cellular radiosensitivity; and (3) Nuclease and radiation-induced DNA DSBs and modulation of EGFR and IGF-1R expression are by quantifying EGFR and IGF-1R expression levels in cells provided by Project 3 that express endonuclease generated DSBs, by pharmacological and genetic manipulation with DN mutants of ATM and DNA-PK to establish the role of these signaling pathways in regulating EGFR and IGF-1R expression after DSB generation and test for a role for P53 in the ATM/DNA-PK to receptor signaling pathway in studies with isogenic P53 wild type and P53 knockout cells.